Organismal traits such as ecological specialization and migratory behavior may affect colonization potential, population persistence, and degree of isolation, factors that determine the composition and genetic structure of communities. However, studies focusing on community assembly rarely consider these factors jointly. We sequenced 16 nuclear and one mitochondrial genes from Caucasian and European populations of 30 forest-dwelling avian species that represent diverse ecological (specialist-generalist) and behavioral (migratory-resident) backgrounds. We tested the effects of organismal traits on population divergence and community assembly in the Caucasus forest, a continental mountain island setting. We found that (1) there is no concordance in divergence times between the Caucasus forest bird populations and their European counterparts, (2) habitat specialists tend to be more divergent than generalists, and (3) residents tend to be more divergent than migrants. Thus, specialists and residents contribute to the high level of endemism of Caucasus forest avifauna more than do generalists and migrants. Patterns of genetic differentiation are better explained by differences in effective population sizes, an often overlooked factor in comparative studies of phylogeography and speciation, than by divergence times or levels of gene flow. Our results suggest that the Caucasus forest avifauna was assembled through time via dispersal and/or multiple vicariant events, rather than originating simultaneously via a single isolation event. Our study is one of the first multi-locus, multi-species analyses revealing how ecological and migratory traits impact the evolutionary history of community formation on a continental island.